1. Field of the Invention
The present invention is related to an improvement in antistatic floor coverings and a method for making the same. It is more particularly related to composite articles, typically floor coverings underlaid with a static electric dissipating substrate.
2. Description of the Prior Art
It is common knowledge that, under conditions of low humidity, especially in winter, surfaces of various floor coverings and the like become dry and, accordingly, have a very high electrical insulating value. As an individual walks over these dry surfaces, rubbing such with the soles of shoes, electrons from the shoes are deposited onto the flooring surface. Depending on the conditions and materials involved, the reverse phenomenon may also occur and electrons from the flooring surface may be deposited on the shoe material. As a result, the individual's shoes exhibit a deficiency or abundance of electrons. Accordingly, a charge is created on the individual and, as this charge is separated from the equal but opposite charge on the surface of the floor covering (as a result of the individual's walking from place to place thereon), work is done which increases the potential or voltage between the individual and the floor covering surface. This action can build up a voltage on the individual of many thousands of volts, which is sufficient to develop a spark whenever the individual comes within the vicinity of a grounded conductor. Irrespective of the physical explanation of the phenomenon of static electricity, it is commonly known that an individual walking over a surface of raised floor coverings during times of low humidity generates a sparking voltage which is discharged when the individual touches a grounded object. Such voltages, of course, depend on the surface covering over which the individual is walking, but have been known to be up to 11,000 volts. The generation of this sparking voltage on the individual is not only annoying, but it is also dangerous since the spark caused by such high voltage discharge may also cause explosions, fires, and the like.
It is known to those skilled in the art that some degree of protection against this build-up of static charge induced by the abovementioned friction on surfaces of floor coverings can be achieved by applying thereto anti-static compounds. Such greatly aids in the dissipation of the static charge, however, it is well recognized that only partial and temporary protection results. The anti-static agents are subject to removal by abrasive forces, i.e. by the individual's walking over the surface of the flooring material, as well as by various cleaning compositions used in cleaning the flooring surfaces. As a result, these compounds must be frequently reapplied. The coatings also have been known to undesirably affect the appearance and/or feel of the floor covering. In some cases, an increase in the apparent soiling rate is noted as a result of the application of these anti-static agents.
Various techniques have been devised for the elimination of such anti-static agents so that a more permanent effect related to static discharge is achieved. Generally, these techniques are dependent on the making of the upper surface of a carpet or other floor covering electrically conductive by incorporating into the tufts or surfaces of the floor covering conductive or sem-conductive elements which extend through the floor covering into an electrically conductive sheet under the flooring material. This conductive sheet, being grounded, allows any potential generated on the surface of the floor covering to move relatively easily through the electrically conducting elements into the electrically conducting sheet and then to the ground. See for example U.S. Pat. No. 2,302,003. While quite effective, these techniques require that the fabric of floor coverings be especially constructed to include a relatively large number of these conducting elements. Metal wires forming the conducting elements and incorporated into the flooring materials not only add to the cost of such, but also being unlike the other materials in the flooring, tend to wear at a different rate and become deformed with time. This causes an unpleasing appearance and is, therefore, objectionable. In addition, conducting or semi-conducting wires extending upwardly through the flooring have a tendency to corrode or bend downwardly out of engagement with the upper surface of the floor coverings and are, therefore, completely useless in terms of any charge dissipation. Finally, special types of conductive backings are required in order to achieve the full effect of static dissipation.
In cases of non-carpet application, the face of ceramic or plastic tiles underlaid by various resilient materials have been continuously coated with a thin metallic film such having embossed into a grid-like pattern, see for example U.S. Pat. No. 2,734,007. While effective in environments that are not subject to high traffic, such as operating rooms, laboratories, and the like, these thin metallic films do not withstand normal traffic wearing readily. Furthermore, the electrical hazards associated with such articles of manufacture is undesirable and any conventional large-scale practical use of such materials is not possible.
Somewhat similar to the above-mentioned superstrate applied to the tile, rather than a continuous metal coating applied to the upper (wear) surface of an article, U.S. Pat. No. 3,713,960 discloses a tufted pile product having anti-static properties which includes a primary backing material through which is bonded the tufted pile and has attached thereto a continuous conductive metal foil. This floor covering is also effective for the dissipation of static charges. However, safety hazards associated with such metal foil underlayment carpets are severe. Direct electric grounding is easily achieved, for example, by spilling water on the surface of the article. In high humidity areas, homes having rooms carpeted with such materials become high shock-risk areas merely from the presence of high voltage sources, such as television receivers, electric ranges, and the like if electrical faults exist or develop. While substitution of a semi-conductive material such as carbon-filled coatings avoids the majority of shock hazards associated with the completely conducting substrate, high humidity conditions can alter the semi-conducting nature of these coatings dramatically, changing an essentially non-conductive coating into one that is conducting.